The present invention relates to a holographic recording and reconstructing apparatus and a holographic recording and reconstructing method for recording and reconstructing information using a hologram and, in particular, to a holographic recording and reconstructing apparatus and a holographic recording and reconstructing method using a reflection hologram.
Recently, holographic recording and reconstructing apparatuses for recording data using holography have been developed.
In holographic recording, a modulated signal beam (i.e., light overlaid with data) and a non-modulated reference beam are generated from a laser beam. These two types of beams are incident onto the same location on a hologram recording medium. As a result, the signal beam interferes with the reference beam so that a diffraction grating (hologram) is generated at the irradiation point. Thus, data is recorded on the hologram recording medium.
By emitting the reference beam to the hologram recording medium, the recorded diffraction grating generates a diffracted beam (reconstructed beam). Since this reconstructed beam contains the data overlaid on the signal beam during recording, the recorded signal can be reconstructed by receiving the reconstructed beam using a light receiving element.
In some cases, to record a large amount of information on a hologram recording medium, a large number of holograms are formed on the hologram recording medium. In these cases, the holograms are not limited to being formed at different locations on the hologram recording medium. The holograms may be formed on the same location, or the holograms may be formed on the hologram recording medium so that parts of the holograms are overlaid. This is known as “angle multiplexing recording”. Examples of the multiple recording method include a variety of recording methods, such as an angle multiplexing recording method, a multi-wavelength recording method, a rotational multiple recording method, and a collinear recording method.
For example, in an angle multiplexing recording method, a plurality of holograms is formed by a reference beam incident on the same location on a hologram recording medium while changing the angle of incidence. When reconstructing the holograms, by using a reference beam similar to a reference beam during recording, a reconstructed beam corresponding to each hologram (i.e. data) can be obtained.
Recently, a holographic recording apparatus that adopts a phase correlation multiple recording method, which is one of multiple recording methods, has been developed to increase the storage capacity (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-242424).
A reflection hologram recording medium provides a variety of advantages over a transmission hologram recording medium. For example, a recording apparatus for a reflection hologram recording medium is expected to have a simpler optical system than that of a recording apparatus for a transmission hologram recording medium. In addition, a reflection hologram recording medium can use an existing servo technology, since the reflection hologram recording medium has an optical system similar to that of existing optical discs, such as a compact disc (CD) or a digital versatile disc (DVD). However, when a reflection hologram recording medium is used, the following four types of holograms are generated: a transmission hologram caused by interference between a signal beam and a reference beam, a transmission hologram caused by interference between a reflected signal beam and a reflected reference beam, a reflection hologram caused by a reflected signal beam and a reference beam, and a reflection hologram caused by a signal beam and a reflected reference beam.
The transmission hologram is formed in a direction perpendicular to the surface of the medium while the reflection hologram is formed in a direction parallel to the surface of the medium. Accordingly, for a medium that is subject to a dimension change, such as reduction, during recording (e.g., photopolymer) , output angles of beams from the two holograms are shifted with respect to each other, and therefore, noise is generated.
Additionally, since the selectivities of the two holograms are different, the signal-to-noise ratio (SNR) is decreased. Accordingly, to obtain superior hologram reconstruction, it is desirable that a diffracted beam from the reflection hologram is removed to obtain only a diffracted beam from the transmission hologram.
To remove a diffracted beam from the reflection hologram, a λ/4 wavelength plate may be disposed between a reflecting film and a recording layer of the hologram recording medium so that a transmission beam and a reflection beam are polarized at right angle to each other. Thus, the interference is prevented. However, this method may increase the cost of the medium.
Accordingly, there is a need for a holographic recording and reconstructing apparatus and a holographic recording and reconstructing method capable of preventing the occurrence of a diffracted beam from a reflection hologram and obtaining only a diffracted beam from a transmission hologram so as to increase the SNR.